Stabilization of acylated or alkylated polyoxymethylenes with tertiary alkanol amineesters



United States Patent 5 Claims. of. 260-4535) The present inventionrelates to normally solid filmforming acylated and/or alkylatedpoly-oxymethylenes of high molecular weight which are stabilized bymeans of organic nitrogen compounds.

w -Dihydroxy-polyoxymethylenes of high molecular weight only show aslight thermal stability in their thermoplastic range between 170 and200 C. A degradation of the high-molecular chain molecules takes placeto give substantially lower molecular weights with formaldehyde beingsplit off mainly from the ends of the chains; a result is the loss ofthe valuable physical properties and the intrinsic viscosities fallsubstantially to values 0.4 (measured in dirnethyl formamide at 150 C).

It is already known to add hydrazines, hydrazides, phenols, aromaticamines, urea and thiourea derivatives to thew,w'-dihydroxypolyoxmethylenes for improving the thermal stability (seeUS. Patent specification No. 2,810,- 708, Belgian Patent 558,777 andBritish Patent Specification No. 748,856). It is true that thethermostability of the w,w'-dihydroxy-polyoxymethylenes is improved inthis way, but the degradation and the quantity of formaldehyde liberatedwhen processing at temperatures above 180 C. is still so considerablethat so far it has not been possible to effect a processing tohigh-grade plastics.

Further improvement in the thermostability of thew,w'-dihydroxy-polyoxymethylenes can be obtained by the terminalhydroxyl groups of the polyoxymethylenes being acylated or alkylated. Inthis way, the amount of formaldehyde split off from the ends of thechains at temperatures above 180 C. is substantially reduced. The amountof formaldehyde split off by thermal cracking of the molecule, which isincreased by traces of acids and impurities, and the degradationrecorded in this case, is however always still sufficient tosubstantially impair the shaping and the physical properties of thepolyoxymethylones of high molecular Weight. Thus, polyoxymethylenediacetates and diethers often show at 220 C. and Within the first 5minutes, decomposition speeds which result in 0.3 to 0.4% offormaldehyde being split off per minute. At lower temperatures, forexample at 190 C., the products certainly split off a smaller quantityof formaldehyde, but the moulded elements manufactured at thistemperature are however permeated by gas bubbles. Furthermore, on beingmelted, particularly in the presence of air, the molecule is degraded toabout of the original molecular weight. As a consequence, the mechanicalproperty values, such as impact bending strength and toughness, areappreciably lowered.

In French Specification 1,131,939 there are disclosed polyoxymethyleneswhich have added thereto antioxidants such as phenyl-fi-naphthylamine or2,6-di-tertiary-butylp-cresol. These additiveshave the drawback thatthey either give rise to a strong discoloration of the compositions orthat they do not stabilize compositions to a degree which is required bythe practice. In French specification 1,179,857 there are disclosedacylated or alkylated polyoxymethylenes of high molecular weight whichare stabilized by addition of polyamides.

It has now been found that the thermal stability of 3,296,194 PatentedJan. 3, 1967 "Ice normally solid film-forming acylated or alkylatedpolyoxymethylenes can be essentially improved by adding to saidpolyoxymethylenes at least one organic nitrogen compound out of thefollowing groups of compounds:

(1) Amines and hydrazines of the general formulae:

NZRI

II R2 X in which X represents a radical comprising an ester group, etherand thioether group, carbonamide group, urethane group, acetal group ornitrile group, bonded to the nitrogen atom by way of an aliphaticradical, or an organic radical, preferably a hydrocarbon radical bondedto the nitrogen atom by way of a Si atom; and R R and R each representaliphatic, cycloaliphatic or -araliphatic radicals or like or dilferentsubstituents X, it being possible for two of the substituents R R R alsoto jointly represent chain members of a ring system which can containfurther hetero atoms and/or double bonds.

(2) Amines and hydrazines of the general formulae:

III R; R3

/N R2 and IV R /R,

N-N R2 \R4 wherein R R R and R represent aliphatic, cyclo- -aliphaticand aralipjhatic radicals, and in case of Formula III two of thesubstituents R R R can jointly :be chain members of a ring system whichif necessary can contain further hetero atoms, and the sum of the carbonatoms of all substituents R R R and R is greater than 12, preferably 12to 40.

(3) Salts of inorganic or organic bases and dithiocarbamic acids.

(4) Aromatic and cycloaliphatic primary, secondary and tertiarydiamines, which are alkylated in the ortho positions to the amino group.

(5) The fifth group of the stabilizers according to the inventioncomprises the following classes of compounds: 1,6-tetra-substitutedhydraz-odicarbonamides, hydrazodicarboxylic acid esters with monohydricaliphatic, cycloaliphatic or araliphatic alcohols, which esters containup to 40 carbon atoms, l-substituted hydrazine monothio-semicarbazidemono-carboxylic acid esters, 1,4-substituted semi-carbazides andthio-semi-carbazides, monosubstituted urethanes and thio-urethanes,allophanic acid esters, biuret derivatives, N-substituted melaminescontaining NH groups, substituted guanidines, formamidines, and amidinesas well as iminoethers, the preferred substituents thereon being alkyl,cycloalkyl, aryl and arelkyl groups having together up to 40 carbonatoms.

(6) Aldehydes of tertiary aromatic amines or their functionalderivatives, such as for example acetals, azines, hydrazones, ox-imes,semi-carbazones, Schifis bases and others.

Examples of suitable stabilizing agents of the recited group 1 ofcompounds are the following:

(la) Esters of silicic, phosphoric, carbonic acid or of saturated orunsaturated aliphatic, cycloaliphatic, aral-iphatic or aromaticcarboxylic acids (having up to 25 carbon atoms) with tertiary amines orhydrazines which are substituted at at least one nitrogen atom by atleast 3 one hydroxalkyl group, the other substituents being alkyl,cycloalkyl or aralkyl radicals (the sum of the carbon atoms in saidamines or hydrazines being up to 54).

These compounds may be represented by way of example by the followingformula:

in which X stands for Si, O=PE or O=C- A is a bivalent aliphatic radicalhaving 2 to 3 carbon atoms (-CH2.CHz-, CHg.CHz.CH3-, CE|.(5H.CH2)

n is a whole integer from 2 to 4 corresponding-to the valence of X, R isa like or different monovalent hydrocarbon radical (alkyl having 1 to 20carbon atoms, cycloalkyl such as cyclohexyl, aralkyl such as benzyl).

in which X stands for the radical:

A.O.(A.O) ,,.CO.R

(A having the same meaning as above, It standing for a whole integerfrom to 3 and R for alkyl having 1 to 20 carbon atoms, cycloalkyl suchas cyclohexyl, aralkyl such as 'benzyl and aryl such as phenyl, tolyl) Rand R are alkyl, cycloalkyl, aralkyl (as nearer defined above) or X andR and R together may stand for members necessary to close a heterocyclicring such as a morpholine,

thiomor-pholine ring.

/N.A.O.(A.O)n.CO =RI s 2 in which R and R have the same meaning as abovewith the variation that they cannot stand for X A and n have the samemeaning as above and R stands fora bivalent hydrocarbon radical(saturated or unsaturated alky-lene having 1 to 12 carbon atoms,phenylene,'cyclohexylene) in which X has the same meaning as above and RR and R are monovalent hydrocarbon radicals as indicated above or XAmong the compounds corresponding to the last three formulas those arepreferred which contain more than 12,

preferably 20 to 36 carbon atoms.

(1b) Ethers and polyethers of saturated or unsaturated aliphatic,cycloaliphatic, araliphatic or aromatic hydroxy compounds includingpolyglycols (having up to 30 carbon atoms) with tertiary amines orhydrazines which are substituted by at least one hydroxyalkyl group(having preferably 2 to 3 carbon atoms) the other substituents beingalkyl, cycloalkyl or aralkyl radicals (all substituents having up to 54carbon atoms).

(1c) Compounds of the Formulas I and II in which X stands for one of theradicals:

R1: 1: -CHz.CO-N CHLOE N OHI-G(CH3).CO.N

I Ria R13 2: wherein R and R stand for hydrogen, aliphatic,cycloaliphatic or araliphatic radicals having up to 26 carbon atoms.

(1d) The reaction products of isocyanates with hydroxyalkylated tertiaryamines and hydrazines corresponding to Formulas I and II in which Xstands for the radical -A.O.CO.NH.R wherein A has the same meaning asabove and R stands for a monovalent hydrocarbon radical (alkyl having 2to 18 carbon atoms, aryl such as phenyl, tolyl, naphthyl, aralkyl suchas benzyl, cycloalkyl such as cyclohexyl), furthermore the reactionproducts of 1 mole of an aliphatic, cycloaliphatic or araliphaticdiisocyanate (having 6 to 10 carbon atoms) with 2 moles of amonohydroxyalkylated tertiary amine or hydrazine as defined above.

(1e) Acetals of the above defined hydroxyalkylated tertiary amines andhydrazines with aliphatic aldehydes having 1 to 7 carbon atomscorresponding to the Formulas I and II wherein X stands for the radical-A.O.B.O.R wherein A has the same meaning as above, B stands for abivalent aliphatic radical having 1 to 7 carbon atoms and R stands foralkyl (having 1 to 20 carbon atoms), aralkyl cycloalkyl.

(1f) Compounds of the Formulas I and H wherein X stands for the radicalsCH .CH CN and -OH20H0N (1g) Compounds of the Formulas I and II wherein Xstands for the radical resulting by subtraction of X from the compoundsof Formulas I and II. Those compounds are preferred which contain onlyone silicon atom.

Specific examples of stabilizers in the group according to the inventionare N,N-dibutyl-trimethylsilylamine, N-methyl-N-octadecyl-trimethylsilylamine,N,N,N',N'-tetran-butyl-dimethylsilyldiamine,dimethylsilyl-bis-N,N'-stearylmethyl-amine, while among the series ofthe ester amines, there are mentioned esters of silicic, phosphoric,carbonic and carboxylic acids, such as for example esters of aceticacid, propionic acid, benzoic acid, malonic acid, succinic acid,phthalic acids with hydroxyethylated or hydroxypropylated tertiaryaliphatic, cycloaliphatic, araliphatic or heterocyclic amines, or thecorresponding, hydrazines, such as for example the esters oftriethanolamine, N-methyl-diethanolamine, N,N,-di-n-butyl-ethano1- amineand N,N-stearyl-methyl ethanolamine (N-methyl- N-stearyl ethanolamineacetate or propionate, or the corresponding n-propanolor isopropanolamine derivatives, N-lauryl-N-butyl ethanol (propanol) amine acetate orpropionate, N-cyclohexyl-diethanolamine diacetate or propionate,N-hydroxymethyl-morpholine acetate or propionate,N-hydroxyethyl-piperidine acetate or propionate,N,N'-dibenzyl-N,N-di-/3-acetoxyethyl hydrazine,N,N'-dicyclohexyl-N,N'-di- 3-acetoxyethylhydrazine). Moreover reactionproducts from the said acids with hydroxyalkyl amines containing ethergroups in the alkyl chain which can for example be obtained by theaction of ethylene oxide or propylene oxide on primary and secondaryaliphatic, cycloaliphatic and araliphatic amines. Other ether amines ofthis group are the reaction product of phenol, guiacol or resorcinolwith 'N,Ndiethylaminoethyl 7 example orthoformic acid esters.

hexy1-, phenyl-, benzyl isocyanate, tetra-, hexamethylene-,1,4-cyclohexyl diisocyanate, 4,4-dicyclohexyl methane diisocyanate) withaminoalcohols such as for example N- methyl-diethanolamine orN,N-dibut'ylethanolamine or the corresponding propanolamine derivatives,whereby the polyisocyanates are reacted with mono-alkanolamines, theproportions being such that the reaction products do not contain freehydroxy or isocyanate groups,

Examples of compounds of the second group are N,N- diethyl stearylamine,N,N-dibutylstearylamine, N,N'-dibenzyl-N,N'-diethyl hydrazine,N-stearylmorpholine, N-

propyl-p-phenylene diamine, 2,3,5,6-tetra-tertiarybutyl-pphenylene-diamine, 3,3',5,5-tetraisopropyl-4,4' diaminodiphenylmethane and the corresponding perhydrogenated, per-alkylatedderivatives.

Examples of the stabilizers of the fifth group are N,N, N",N""-tetra npropyl hydrazodicarbonamide, N,N, N"",N""-tetra-n-butylhydrazodicarbonamide, hydrazodicarboxylic acid dipropyl ester,hydrazodicarboxylic acid dibutyl ester, N,N-dimethylhydrazine carboxylicacid lauryl ester, reaction products of hexamethylene diiso cyanate orhexamethylene diisocyanate with monoand polyglycols or mercaptans,1-phenyl-4-dibutyl-semicarbazide, 1,4-diphenyl thiosemicarbazide,trimethylol-melamine tri-n-butyl ether, N-phenyl- ',N',N",N"-tetramethylguanidine, N,N-N"-triphenyl guanidine, N,N' N" triphenyl guanidine,N-phenyl-N,N'-dimethyl formamidine, N,N-diphenyl benzamidine andbenziminoethyl ether.

Examples of the stabilizers of the sixth group are4-diethylaminobenzaldehyde, 4 dimethylaminobenzaldehyde azine (reactionproduct of 2 mols of the aldehyde with 1 mol of hydrazine),4-diethy1aminobenzaldehyde-phenyl hydrazone, 4-diethyl-aminobenzaldehydephenyl hydrazone and 4 diethylaminobenzaldehyde phenyl semicarbazone.

The aforesaid compounds are suitable for stabilizing normally solid,film-forming polyoxymethylenes of high molecular weight having terminalacyl or ether groups, such as acetyl, propionyl, stearoyl, benzoyl, acylgroups derived from cyclohexyland phenyl acetic acid (that is to sayacyl groups derived from aliphatic, cycloaliphatic araliphatic andaromatic carboxylic acids), and alkyl ether groups having preferably 1to 4 carbon atoms. For

' producing these products the normally solid polyoxymeth ylenes may beacylated by means of acetic acid, propionic acid, benzoic acid or othercarboxylic acids and/or may be alkylated with the aid of ort-ho-esters,such as for Such acylated and/or alkylated polyoxymethylenes of highmolecular weight are for example described in Belgian patentspecification 583,933, and in the specifications of the US. patentapplications 1,856 filed Dec. 1, 1960 and 13,708 filed Sept. 3, 1960.

The stabilizers according to the invention are added to thepolyoxymethylenes stabilized at the terminal groups in quantities ofabout 0.01 to 4 percent by weight, adv-antageously 0.5 to 3 percent byweight, prior to the processing and shaping, it being possible for thestabilizers to be used singly or in any desired mixture. It is moreoverpossible to use them in combination with known antioxidants, such as forexample phenols, sulfur-containing compounds such asmercaptobenzthiazole, and also with fillers, such as carbon black,plasticizers, lubricants, inorganic and organic pigments and otheradditives.

The stabilizers or mixtures of these stabilizers can be added insubstance to the recited acylated or alkylated polyoxymethylenes bymilling or kneading, but they are advantageously dissolved in a solventand sprayed on to the products or intimately mixed with excess solventstogether with the polyoxymethylenes stabilized at the terminal groups,the solvent thereafter being evaporated.

The stabilizers according to the invention permit more especially aprocessing of the high molecular weight polyoxymethylenes stabilized atthe terminal groups in accordance with the injection moulding process,without the molecular weights of the said polyoxymethylenes falling intoranges which are characterized by brittleness and low strength values.

The stabilizing action of the stabilizers according to the invention isto be shown by reference to the following comparison: Whereas apolyoxymethylene diacetate with an intrinsic viscosity of 0.85 (measuredat a 0.5 percent solution in dimethyl formamide at 150 C.) on beingmelted while in contact with air at 200 C. during a heating period of 2minutes, changes into a thinly. liquid melt which, after cooling, yieldsa brittle film which breaks easily, the said film consisting ofpolyoxymethylenes which only still have an intrinsic viscosity of 0.2,the same sample with the same intrinsic viscoisty, to which for example0.5 percent of triethanolamine triacetate, N,N-dibutyl ethanolamineacetate or N-methyl-diethanolamine diacetate are added, produces anintrinsic viscosity of 0.59

to 0.65 when treated in the same manner; the films thereby obtained aretough, pliable and elastic. If the same experiment is carried throughwhile using the same amount of a high molecular weight poly-amide(produced from (ecaprolactame), the viscosity value drops to 0.42. Bythe addition of the stabilizers according to the invention, themolecular weight ranges with the valuable physical properties(corresponding to intrinsic viscosities of 0.55 to 3.0 as measured at0.5 percent solutions in dimethylformamide at 150 C.) are substantiallymaintained at the high processing temperatures.

In the following examples, the parts indicated are parts by Weight.

EXAMPLE 1 Each batch comprising 10 parts of high-molecular acetylatedpolyoxymethylene having an intrinsic viscosity of 0.85 (measured at 150C. at a 0.5 percent solution in dimethyl formamide), is suspended in 60to parts by volume of acetone and the mixture has added thereto 0.4percent of beeswax and one of the following stabilizers in quantities of2 percent (:02 part): triethanolamine triacetate,N-methyl-diethanolamine diacetate or dipropionate,N,N-dibutyl-fl-ethanolamine acetate or propionate,N-methyl-N-stearylamino-propionitrile, silicicacidtetra-(fi-N-dibutylaminoethyl)-ester, dimethylsilyl-bis-(N-butylamine), phosphoric acid-tris- (fi-N-dibutylaminoethyD-ester,N,N-dibutyl-N,N-di-B acetoxyethylhydrazine,N-methyl-N-stearyl-methylstearyl ammonium dithiocarbamate,dimethylstearylamine, trimethyl stearylamrnonium acetate,2,3,5,G-tetraethyl-p-phenylene diamine, 1,6tetra-n-propy-hydrazodicarbonamide, 1,4 diphenyl thiosemicarbazide,N,N-dimethylhydrazine carboxylic acid lauryl ester,p'dimethylaminobenzaldehyde phenyl hydrazone, fi-diethyl aminoethyl,phenyl ether or 3,3',5,5'-tetraisopropyl-4,4'-diamino-diphenylmethane.

While stirring, the acetone is slowly evaporated, the polyoxymethylenemixture is shaken for 10 minutes, thereafter dried and once againthoroughly mixed. Two parts of the unstabilised specimen and each of thestabilised specimen are melted in air at 200il in a thin layer andquickly cooled after 2 minutes. Whereas the melt of the unstabilisedspecimen splits oir' formaldehyde to an appreciable degree and the meltviscosity rapidly falls, the stabilised specimens show only a slight orno erization vessel.

formaldehyde vapour pressure and remain viscous; in contrast to theunstabilised comparison specimen, they are tough and elastic aftercooling. The following table shows the extreme drop in the intrinsicviscosity of the unstabilised specimen, in comparison with a number ofstabilised specimens.

Specimens i before m after Properties melting meltingUnstabilisedpolyoxy-methylene 0.85 0. 21 Brittle, no

diacetate. strength. +triethanolamine triacetaterfl. 0. 85 0. 61Elastic, tough. +N-methyl-diethanolamine 0. 85 O. 58 Do.

diacetate. +N-methyl-N-stearylmethyl- 0. 85 0. 57 Do.

stearyl ammonium dithiocarbamate. 3,3,5,5-tetraisopropyl-4.4- 0. 85 0.64 D0.

diaminodiphenylmethane.

With an inlet pipe for formaldehyde, a mechanical stirrer and agas-outlet pipe. .Disposed in the polymerization vessel are 1000 partsby volume of anhydrous toluene, to which are added 0.035 part by weightof tetramethyl urea, dissolved in 3 parts by volume of anhydroustoluene. The formaldehyde is added dropwise over a period of 2 /2 hoursand while stirring into the reaction medium cooled to 20 C., wherebypolymerization takes place. After stirring for another hour at -20 C.,the polymerization product is suction filtered. There is obtained a purewhite high molecular weight polyoxymethylene,

which is extracted by stirring twice with methanol and twice withacetone. vacuo for 4 hours at 60 C. The yield is 118 g.

Thepolyoxymethylene thus obtained is acetylated according to thefollowing prescription:

20 parts of the recited polyoxymethylene are treated With400 parts ofacetic anhydride, 20 parts of phenyl isocyanate (or parts oft'olyl-2.4-diisocyanate) and 0.7

part of sodium acetate in' a nitrogen atmosphere for 15,

hours at 139 to 140 C. The acetylated polyoxymethyl- The polymerizationvessel is provided The product is thereafter dried in' is determined at222 C. Whereas the unstabilized speci- 1 men has already split off 5.4percent of formaldehyde after minutes and 14.5 percent after 120minutes, the stabilized specimen loses only 3 percent of formaldehyde in20 minutes and only 4.6 percent after 120 minutes.

The polyoxymethylene may be prepared by polymerizing substantiallyanhydrous formaldehyde in an inert solvent while using aluminum oxide ascatalyst (compare French Specification 1,226,239). The acetylation maybe carried through according to the prescription of EX- ample 1 With thevariation that diisopropyl carbondiimide is used as catalyst.

EXAMPLE 4 A high molecular polyoxymethylene diacetate may be preparedaccording to the data of Example 3 and has an intrinsic viscosity of0.75 at 150 C. at a 0.5 percent solution in dimethyl formaimide, ismixed according to Ex? ample 1 with 1.5 percent of 1,6-tetrapropylhydrazodi-,

carbonamide and 0.5 percent of beeswax and the thermostability of thespecimen is determined at 222 C. under a nitrogen atmosphere. Whereasthe unstabilized'specimen has already lost 12 percent of formaldehydeafter 20 minutes and 18 percent after 120 minutes, the stabilizedspecimen loses 6 percent of formaldehyde in 20 minutes and 10 percentafter 120 minutes.

EXAMPLE 5 An ethylated high-molecular polyoxymethylene as obtainedaccording to Example 1 of Belgian Specification 583,593 is mixedaccording to Example 1 with 1.5 percent ofN-methyl-N-stearyl-methylstearyl-ammonium dithiocarbamate and'thethermostability is determined at 222 C. under nitrogen. Whereas theunstabilized specimen has already lost 37 percent of formaldehyde after20 minutes and 79 percent thereof after 120 minutes, the

stabilized specimen loses only 9 percent of formaldehyde in 20 minutesand 26.6 percent after 120 minutes.

EXAMPLE 6 An acetylated polyoxymethylene is used which has an intrinsicviscosity of 1.0 (measured at 0.5 percent solution in butyrolactone at150 C.) 100 parts of the poly-; oxymethylene diacetatev are suspended ina solution of 0.8 part of bis-thiourethane obtained from 1 mol ofhexamethylene diisocyanate and. 2 mols of dodecyl mercaptan and 0.3 partof beeswax in acetone. Thereafter, the. acetone is completely evaporatedin vacuo and the specimen'is finally dried at 60 C. in vacuo. Thethermostaene is filtered off from the cooled reaction solution, freedfrom acetic anhydride by washing several times with acetone andmethanol, freed from traces of sodium acetate by washing with water, anddried after a further treatment with acetone.

EXAMPLE 2 In a manner analogous to Example 1, 2' percent oftriethanolamine triacetate are admixed with an intrinsic viscosity of1.2 (measured at a 0.5 percent solution in dimethyl formamide at 150 C.)and the thermostability of the specimen is determined at 2220 C. under anitro-' gen atmosphere. Whereas the unstabilized specimen has alreadylost 5 percent of formaldehyde after 20 minutes and 16 percent thereofafter 120 minutes, the stabilized specimen only loses 1 percent offormaldehyde in 20 minutes and only'5 percent after 120 minutes.

EXAMPLE 3 An acetylated polyoxymethylene of high molecular weight,having an intrinsic viscosity of 0.95 (measured at a 0.5 percentsolution in dimethylformalmide at 150 C.),

is mixed in accordance with Example 1 with 1.2 percent ofN,N-dibutyl-ammonium dithioc'arbamate and 0.5 per cent of beeswax andthe thermostability of the specimen bility is determined at 222 C. undernitrogen. Whereas an unstabilized comparison substance has already splitoff 8 percent of formaldehyde after 20 minutes and 20 pera cent after120 minutes, the loss of formaldehyde with the stabilized specimen after20 minutes is only 4 percent and after 120 minutes it is only 7 percent.

EXAMPLE 7 The procedure indicated in Example 6 is followed and apolyoxymethylene diacetate is used which has an intrinsic viscosity of1.8 (measured at a 0.5 percent solution in butyrolactone at 150 (3.).The additives admixed in acetone solution with thepolyoxymethylenediacetate amount to 0.7 percent of 'N-methyl-N-stearylt methylstrearylammonium dithiocarbamate and 0.3 percent of beeswax. bility carried outunder a nitrogen atmosphere at 222 C. show that an unstabilizedcomparison specimen splits off 14 percent of formaldehyde after 20minutes and 37 percent thereof after 120 minutes, whereas the stabilizedpolyoxymethylene diacetate only loses 2 percent of form aldehyde after20 minutes and only 7.5 percent thereof after minutes.

EXAMPLE 8 The procedure is as indicated in Example 6. A polyoxymethylenediacetate is used which has an intrinsic viscosity of 0.72 measured inbutyrolactone at C.). The

The measurements of the thermosta- The procedure is as indicated inExample 6 and a polyoxymethylene diacetate is used which has anintrinsic viscosity of 1.1 (measured at 0.5 percent solution inbutyrolactone at 150 C.). polyoxymethylene diacetate in acetone solutioncomprise 2 percent of 2,3,5,6,-tetraethyl-p-phenylene diamine and 0.5percent of beeswax. The thermostability measurements carried out under anitrogen atmosphere at 222 C. show that an unsta bilized comparisonspecimen splits off 6 percent of formaldehyde after 20 minutes and .13percent thereof after 120 minutes, while the stabilized polyoxymethylenediacetate loses only 2 percent of formaldehyde after 20 minutes and only5 percent after 120 minutes.

EXAMPLE The procedure is as indicated in Example 6 and apolyoxymethylene diacet-ate is used which has an intrinsic viscosity of1.0 (measured at 0.5 percent solution in butyrolactone at 150 C.). Theadditives admixed with the polyoxymethylene diacetate in acetonesolution comprise 2.5 percent of diethyl stearylamine and 0.4 percent ofbeeswax. The thermostaibility measurements carried out under a nitrogenatmosphere at 222 C. show that an unstabilize-d comparison specimenalready splits off 10 percent of formaldehyde after 20 minutes and 25percent thereof after 120 minutes, while the stabilized polyoxymethylenediacetate loses only 3 percent of formaldehyde after 20 minutes and only8 percent thereof after 120 minutes.

EXAMPLE 1 1 The procedure is as indicated in Example 6 and apolyoxymethylene diacetate is used which has an intrinsic viscosity of1.4 (measured in butyrolactone at 150 C.). The additives admixed withthe polyoxymethylene diacetate in acetone solution comprise 2 percent of4-diethylaminobenzaldehyde phenyl hydrazone and 0.4 percent of beeswax.The thermostability measurements carried out in a nitrogen atmosphere at222 C. show that an unstabilized comparision specimen already splits oif5 percent of formaldehyde after 20 minutes and 24.5 percent thereofafter 120 minutes, whereas the stabilized The additives admixed with thepolyoxymethylene diacetate only loses 2 percent of formaldehyde after 20minutes and only 6 percent thereof after minutes.

As to the compounds of group 3 of the present specification these maycorrespond to the general formula:

wherein the radicals denoted R are like or difierent monovalentaliphatic radicals (having 1 to 20 carbon atoms), cycloaliphaticradicals (such as cyclohexyl), araliphatic radicals (such as benzyl),and both radicals may form members of a heterocyclic ring (such as ofpiperidine or morpholine), Me stands for a metal or an amine asindicated above and n for a Whole number corresponding with the valencyof Me. As salt forming bases there are preferred secondary amines whichare substituted by aliphatic araliphatic or cycloaliphatic radicalshaving preferably a total of 8 to 40 carbon atoms.

We claim:

1. A composition comprising a thermally stabilized, normally solid,synthetic high molecular weight polyoxymethylene selected from the groupconsisting of acylated and alkylated polyoxymethylenes and a stabilizingamount of a monocarboxylic acid ester of a tertiary ethanolamine.

2. The composition of claim 1 wherein said ester is present in an amountof 0.01 to 4% by weight based on the weight of said polyoxymethylene.

3. The composition of claim 1 wherein said ester is triethanolaminetriacetate.

4. The composition of claim 1 wherein said ester isN-methyl-diethanolamine diacetate.

5. The composition of claim 1 wherein said ester isN,N-dibutyl-p-ethanolamine acetate.

References Cited by the Examiner UNITED STATES PATENTS 2,768,994 10/1956MacDonald 26045.9 2,810,708 10/1957 Kubico et a1. 26045.9 2,920,0591/1960 MacDonald et a1. 260-459 2,966,476 12/ 1960 Kralovec et al260-45.9 2,998,409 8/ 1961 Nogare et al 26045 .9

FOREIGN PATENTS 748,856 5/1956 Great Britain. 1,179,857 12/=195 8France.

LEON I. BERCOVITZ, Primary Examiner.

ALLPHONSO D. SULLIVAN, MILTON STERMAN,

Examiners.

H. LEVINE, H. E. TAYLOR, Assistant Examiners.

1. A COMPOSITION COMPRISING A THERMALLY STABILIZED NORMALLY SOLID,SYNTHETIC HIGH MOLECULAR WEIGHT POLYOXYMETHYLENE SELECTED FROM THE GROUPCONSISTING OF ACYLATED AND ALKYLATED POLYOXYLENES AND STABILIZING AMOUNTOF A MONOCARBOXYLIC ACID ESTER OF A TERITIARY ETHANOLAMINE.